A recent translation included the word iisa. This word might seem like a verb meaning perhaps “join” or “unite” at first glance, and indeed, that’s how I wanted to translate it, despite knowing subconsciously I’d seen it a lot and it might be a special case where reduplication is allowed without changing the meaning much. (Actually, magkaisa or rarely isahin would be “join.”)
Luckily, my wife was very sure I was wrong, so this error did not enter the final draft, but couldn’t explain why. I deferred to native instinct as usual. However, I later found an explanation when I decided to search tatatlo on Glosbe, my new favorite online English-Tagalog dictionary. (Finally, the Watch Tower Bible and Tract Society of Pennsylvania (Jehovah’s Witnesses organization) is good for something, all their translated materials, plus many other public domain translations, make up the database.)
The reason is that isa is a number, and reduplication has a different meaning on numbers, even though numbers have the grammatical function of adjectives in most cases. As Fiona Vos explains, reduplicating the first syllable of a number gives it a meaning similar to “n na lang” (only n); so iisa is “only one”, dadalawa “only two”, tatatlo “only three”, aapat “only four”, and so on, even lilibo, “only one thousand”.
This also finally helps me make sense of the word mag-iisa, which I never understood why it has reduplication despite not being future tense.
A post on a Filipino forum provides a succinct example of this construct:
Di ka na mag-iisa, kasi magdadalawa tayo.
Meaning, “You won’t be alone, because there are two of us.”